Anodes pitting

If the technical regulations are adhered to for constructional steels in neutral waters, there are no conditions for H-induced corrosion. On the other hand, hardened and high-strength materials with hardnesses above HV 350 are very susceptible [60,82,92], since anodic polarization encourages crack formation in saline media and anodic pitting occurs with acid products of hydrolysis [93]. 

Detection is obviously specific to a particular active inhibiting chemical. Some tests are expensive and difficult. Much work has gone on to develop a test for aminoalcohols, with mixed results so far. As stated earlier, it is far easier to carry out test on admixed inhibitors than for those applied to hardened concrete. There can be dangers with under dosing inhibitors, in that corrosion may occur very aggressively in small anodic pits. It is therefore important that the risk of pitting and under dosing is fully understood. 

In Fig. 5 is shown the microscopic images of the surface after exposure in conditions of anode pitting nucleation potential (exposure in Ringer solution). On the material surface there are visible not evenly distributed pits with different sizes. The highest amount of pits is located on the sample III surface. Exposure in the same solution at a lower potential does not lead to the creation of pits. 

The Ti02 film, being an n-type semiconductor, is electronically conductive. As a cathode, titanium permits electrochemical reduction of ions in an aqueous electrolyte. On the other hand, very high resistance to anodic current flow through the passive oxide film (i.e., significant anodic polarization) can be expected in most aqueous solutions. Elevated anodic pitting (breakdown and repassivation) potentials can also be expected with many titanium alloys. 

Pure Bromide/Iodide Solutions anodic pitting aU Dependent on alloy composition and surface, and pH tmd temp. 

The susceptibility of an alloy to pitting on openly exposed surfaces (in the absence of crevices) in a given solution and fixed temperature is normally assessed by determination of anodic pitting potential values. The two pitting potentials of interest are (1) the extrinsic, surface-dependent anodic breakdown potential (Eb), and (2) the conservative, intrinsic anodic repassivation potential (or protection potential (Eprot)) that defines the minimum potential at which pitting is possible in the test environment. 

Pitting and crevice corrosion are types of localized corrosion. This form of corrosion can be observed for instance on passive metals especially austenitic stainless steels in the presence of certain anions (chloride, bromide). Pitting corrosion (Fig. l-9b) results from a local destruction of the protective passive film, with the formation of a small corrosion anode. Pitting can start at faults in the passive film or at non-metallic inclusions like sulfides in stainless steel. Because of hydrolysis of the corrosion prod-... 

Repassivation potentials (Ep) are conservative measures of anodic pitting tendency because they represent minimum potentials below which pitting cannot be sustained. Unalloyed titanium exhibits the highest value, which decreases as... 

Like anodic pitting potentials, repassivation potentials Eire significantly lower in bromide and iodide media. Room-temperature Bp values of+1.2 and +0.95 V are measured for grades 2 and 5 titanium, respectively, whereas values of +0.9 V in dilute KBr solutions have been reported. Repassivation potentials for grades 2 and 5 titanium in dilute room-temperature iodide solutions have... 

Anodic pitting potential values are significantly lower in bromide solutions, and they decrease with increasing temperature. At room temperature, einodic pitting potentials of+0.90 to +1.4... 

However, additions of various oxidizing anions may inhibit pitting in NaBr solutions by significantly raising anodic pitting potentials (Ref 2). Critical concentrations of these inhibitive anions have been determined, and the relative efiiciency of inhibition decreases in the order SOI > NO- 3> CrOl - > POf > COi -. 

Studies in room-temperature iodide solutions have revealed anodic pitting potentials of +1.7 to +1.8 V, with little effect of acidification indicated (Ref 1,4). Above 40 to 50 °C (100 to 120 °F), values near +0.5 V (versus SCE) are reported. 

Because Ti-6246 is less resistant to oxidizing media than CP Ti, it is expected that pitting resistance would likewise suffer. This is indeed the situation observed for repassivation potentials, which represent conservative measures of anodic pitting below which pitting cannot be sustained. 

In sulfate and phosphate media, anodic pitting potentials of titanium alloys are typically in the range of +80 to +100 V (versus Ag/AgCl reference electrode). 

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